\ '! in8 Appendix VL Boosters are motor-dynamos applied to automatically in- crease or ' boost' the voltage of a line already supplied with current. There are two principal uses for boosters: (i) when a storage battery requires high voltage for charging during light load at a station,, and a booster is put in series between the main generator and the cells to raise the voltage to the required amount, as in the figure; (2) when the inevitable voltage drop in long lines is made up in a similar manner. Motor Starters.—On starting a motor the current would reach an excessive value if it were turned on to the motor instantly. To prevent this a multiple-step rheostat is used to keep the current to a reasonable value, the surplus being absorbed by resistances that are cut out one by one till full speed be attained. Referring to Fig. 986, there are three terminals A, B, and c on the switch board, connected respectively to the main supply s, the field winding, and the armature. Also B is coupled to the field switch, and c to the armature switch, each of which is supplied with a number of resistances. In the figure the switch arm is shewn on the vulcanite insulators vv, and current is cut off. Moving the, arm downward, the field winding is gradually supplied as RJ and R2 are cut out, after which the armature resistances R8 to R7 are similarly eliminated, the motor speed being finally regulated on the armature circuit. J?p, 5$$ and 961. Electric Aceumtilators. —Secondary cells have improved considerably; Their efficiency is usually reckoned on< the combined charge and discharge. Stated on what happens to the cell itself, without regard to the engine and dynamo :— f^> rr- • ampere-hours discharge , 0 0/ Current efficiency = ~~^^^^^^ = about 89% T-, n- * Watt-hours discharge , - , y »f Energy efficiency - Watt4^ dEgT - about 60% The reason for the lower result in.the second case is the decrease of voltage per cell from 2*14 to 1*9 volts between charge and discharge. Naturally, however, the second statement is the one having most value for the engineer.